Exploring the Repositories
There are several gui-based methods you can use to explore the repositories. One is to open Synaptic and click on the Origin button. This will display a repository list in the window above the button. The list will contain all the repositories enabled in your sources.list. To the right will be the packages available via download (white boxes) and currently installed packages (green boxes).
There are two images of the main Synaptic page below. The first, on the left, shows the results of selecting local/main. Local packages are packages stored on the user's computer. The second example, on the right, displays the results of selecting the Ubuntu main online repository.

Installing Mozilla's Firefox on Ubuntu
Why would you want to install the Mozilla version of Firefox?
Copy and paste terminal commands
Notes
Commands to remove Mozilla's version
Use a script to automate the commands
Why would you want to install the Mozilla version of Firefox?
There's usually no reason anyone would have to install the Mozilla version of Firefox. The Ubuntu repositories version receives security updates and is integrated with the filesystem. If you use the Mozilla version, you'll have to integrate it with the filesystem so that it launches properly and uses the right multimedia plugins, and you'll also have to do manual security updates.